In recent years, an optically assisted recording scheme has been proposed as a recording scheme for recording data at a density of 1 Tb/in2 or more. In a conventional magnetic recording device, recorded information may be lost due to a heat fluctuation when a recording density is equal to or more than 1 Tb/in2. In order to avoid this, increases in the coercive force of a magnetic recording medium may be used. However, the intensity of a magnetic field produced by a recording head is limited. If the coercive force is excessively increased, recording bits cannot be formed on the recording medium. To solve this, in current devices the medium is irradiated with light at the moment of recording and thereby heated in the optically assisted recording scheme. Heating the medium reduces the coercive force. Thus, data can be recorded on a medium having a high coercive force. Therefore, a recording density of 1 Tb/in2 or more can be achieved.
The diameter of a spot of light with which a medium is irradiated to heat the medium should be close to the size (several ten nanometers) of a recording bit in an optically assisted recording device. This is because when the diameter of the light spot is larger than the recording bit, information stored in an adjacent track is deleted, e.g., it is written over by the new data or erased by the light reducing the coercive force for that bit. In order to heat such a fine region, near-field light is used. The near-field light is an electromagnetic field (light whose wavenumber has an imaginary component) locally present near a fine object smaller than one light wavelength. The near field is generated by using a metal scatterer or fine opening whose diameter is smaller than the light's wavelength. Japanese Patent Office (JPO) Pub. No. JP-A-2007-293972 describes an element that has a narrowed portion made of metal and generates near-field light by irradiating the narrowed portion with laser light.
In order to efficiently generate near-field light, the laser light should be effectively incident on a near-field light generating element such as a fine opening or a metal scatterer. This is because when the diameter of a spot of the incident light is much larger than the size of the fine opening or metal scatterer, a large part of the incident light is not incident on the near-field light generating element and passes through the near-field light generating element. This portion of the light is not transferred to the recording medium at the point desired and results in energy loss and a reduction in the efficiency of the near-field light source.
Therefore, it would be beneficial to optically assisted magnetic recording heads to have a system which causes light to be efficiently incident on a near-field light generating element and thereby more efficiently generates near-field light than conventional systems can due to the problems described previously.